Virtually all forced air systems for heating and cooling interiors of residences or similar buildings include a filter to remove from air being circulated within, or moved into, the interior of the building, dirt, dust and other airborne foreign particles. Such filters are commercially available generally as fiber glass filters which require periodic replacement or they are electrostatic air cleaners which require periodic cleaning. Periodic or a preventive maintenance of a filtration system may be based upon a scheduled event such as annual maintenance of the system or otherwise scheduled. However, more often the filter is changed or cleaned because noticeable reduction in the movement of air through the system and the system does not seem to be working as well as it should. It requires a certain discipline to maintain an on going maintenance scheduled, if not a good memory, to assure proper system operating efficiency. If filters are changed too frequently waste occurs due to the time and effort required to substitute the new filters for the used ones in addition to the costs associated with obtaining and disposing of more filters than necessary and the cost of the filters. Cleaning electrostatic cleaners too often not only requires unnecessary work, but contributes to their wear and tear and to potential damage. However, if the filters are not changed or cleaned at sufficient intervals, operation of the system suffers, becoming less efficient and more costly.
The situation is complicated because filters tend to accumulate material at different rates depending upon the seasons, activities within the buildings and whether the system is being used for heating wherein the blower is generally operated at a low speed or is being operated in the higher speed cooling mode. Accordingly, a "watch dog" type of device that continues to monitor the condition of filtration in a forced air system, irrespective of the variability of airborne dust content and the mode of operation, minimizes operating costs while maximizing operating efficiency. Automatic systems for detecting the clogging air filters are known. They typically involve sensing the pressure drop across the air filter. As the filter becomes increasingly dirty, static pressure across it begins to increase, depending upon the air velocity of the air moving through the duct wherein the air filter is located. A system for detecting a clogged filter in a variable volume system is disclosed in U.S. Pat. No. 4,751,501 which issued Jun. 14, 1988 to E. Gut. This system includes a device which senses the air pressure differential across the filter. The device has an alarm to indicate the need for filter replacement and depends upon a somewhat complex air flow velocity sensor and controller to compensate for changes in air velocity. Such sensor and controller prevent premature filter replacement indication by the device if the differential pressure across the filter increases solely to an increased in the volume of the air moving through the duct. U.S. Pat. No. 3,934,543 which issued to D. Howard on Jan. 27, 1976 is directed to apparatus for monitoring the condition of an air filter comprising a mechanical device which is operated by means of diaphragm and levers to display a "flag" when an increase in the pressure differential across the filter is sensed. The device includes an audible alarm option and adjustment means are provided for the mechanism that indicate the need for replacement. Unlike the device in the above described patent to Gut, however, neither the selector nor automatic provisions are provided to compensate for changes in the volume of air flowing through the duct. U.S. Pat. No. 2,921,157 to J. Stein which issued Jan. 12, 1960 is for a filter gauge that is also a mechanical device which displays a " flag" or causes an electrical indication mechanism to operate when the predetermined pressure differential levels are achieved. The sensitivity of this device is adjustable for specific applications. However, Stein's filter gauge does not include a means for selecting distinct means to indicate different air volume flows and there are also no provisions for automatic compensation of different volume air flows. Devices such as those described above tend to be costly and complex to produce, have more moving parts than desirable for the specific functions and unless cleaned regularly their operations may be constricted or prevented by the accumulation of dust and wear and tear.